The present invention related to a biaxially oriented film having a uniform surface and excellent slipperiness and wear resistance.
Polyester films, which have excellent physical and chemical properties, are widely used as an industrial material. Above all, biaxially oriented polyethylene terephthalate films are used for base films of magnetic recording media and dielectrics of capacitors due to their excellent mechanical strength, dimensional stability and flatness. By utilizing the excellent transparency, they are also widely used in the fields of graphic arts, display, package materials, etc.
In the practical use of these films, they are required to have a god running property and a high wear resistance, but these properties cannot be said satisfactory in conventional polyester films.
For example, when a film is brought into contact with another film or a part such as a metal pin at a high speed, wear is increased, whereby a scratch or abrasion dust is produced on the film. In magnetic recording, for example, the abrasion dust causes a missing in a recording signal, namely, dropout, thereby greatly deteriorating the commercial value of the film.
It is generally known that if the surface of the film is made coarse to a certain extent, running property and the wear resistance are improved. For this purpose, a method of adding fine particles to a material polyester is adopted and partially put to practical use, but the quality of the thus-produced polyester is still insufficient in simultaneous improvement of running property and the wear resistance.
For example, so-called precipitated particles produced from a residual catalyst at the time of producing a polyester are easily broken during the stretching process, and a film obtained by using such particles is inferior in running property and wear resistance, and in addition, is difficult to reuse. On the other hand, when inert inorganic or organic particles are added to and mixed with a polyester, the particles often slip off the surface of the film in the form of a white powdery material.